(1) Field of the Invention
The present invention relates to a method and an apparatus for supplying ozone gas concentrated within a predetermined concentration range to ozone consumption equipment such as semiconductor manufacturing equipment. More particularly, the present invention relates to a method and an apparatus for purifying ozone gas generated by an ozone generator (ozonizer) and supplying the purified ozone gas as an ozone gas concentrated within a predetermined concentration range.
(2) Description of Related Art
Generally, ozone gas is generated by supplying oxygen gas from an oxygen tank or separated from the atmosphere to an ozone generator. However, even when ozone gas is generated by using oxygen gas from an oxygen tank, the generated ozone gas contained in the oxygen gas has a concentration of as low as approximately 5 vol % to 10 vol %. In addition, due to its great self-decomposability, ozone gas is self-decomposing while flowing through an ozone gas supply route. At a stage where the ozone gas is supplied to ozone gas consumption equipment, a concentration thereof is further decreased and the supply-concentration thereof is unstable by nature. In a semiconductor manufacturing field, recently, the oxidation power of ozone has been increasingly utilized to form an oxidation film on a substrate or some other material. In this case, it is desirable that ozone gas to be supplied have a middle level of stable concentration, in order to stably form an oxidation film of an appropriate thickness in a short period of time.
The applicant has previously proposed a technique in which ozone-oxygen mixture gas is supplied from an ozone generator to adsorbing cylinders filled with an ozone adsorbent being in a non-cooled state whereby the ozone gas is selectively adsorbed to the adsorbents, and in which the adsorbing cylinders are vacuumed during a desorption operation of the ozone gas whereby the ozone gas is desorbed from the adsorbents (refer to WO 2008/062534). Furthermore, a technique has been proposed as well, in which an equalizing tank is attached to a derivation passage for ozone gas desorbed from an adsorbing cylinder, and a decompression section is disposed on the downstream side of the equalizing tank (refer to JP-A-61-72602).
The ozone concentration technique disclosed in WO 2008/062534 enables ozone gas to be concentrated until the concentration thereof is substantially tripled, by repeating the adsorption/desorption of ozone gas to/from the adsorbent being in a non-cooled state. According to an experimental example, this adsorption pressure is 3.4 kPa·G, 7.1 kPa·G, or 12.3 kPa·G.
Typically, as the adsorption pressure increases, the adsorbed amount of ozone gas increases. However, when ozone gas is adsorbed at high pressure, the pressure increases rapidly in the concentrated ozone extraction pipe immediately after the desorption starts. With this, the tendency is found out where the ozone is more likely to self-decompose in the pipe or a decompression section (vacuum pump). Therefore, it seems difficult to increase the adsorbed amount, simply by increasing the pressure.
Meanwhile, the technique disclosed in JP-A-61-72602 in which the equalizing tank is attached to the derivation passage for ozone gas desorbed from the adsorbing cylinder, and the decompression section is disposed on the downstream side of this equalizing tank has the following problem. Although it is possible to prevent the rapid pressure increase in the derivation passage for desorbed ozone gas immediately after the desorption of the ozone gas starts, the performance of the decompression generation section cannot be exerted on the desorption operation of ozone gas which is carried out by the adsorbing cylinder. Accordingly, the desorption pressure does not decrease sufficiently, thereby failing to concentrate ozone gas appropriately.